Storing heat in sand?

[MyK3y]

I was thinking ( which is a scary thing ) of building a cob oven insides with fire brick, inside then coating that with cob. Next with about 1 inch of cob on that create a pocket on top of the cob of dry sand. then in the sand put heating elements, then seal all of that in with cob..... Thus you could heat the thermal mass either by the heating elements or use wood heat.... May need chicken wire over the final pat to give it more stucture......

Sure. But how are you insulating that? Or are you making a passive thermal store that will just give up its heat naturally?

I thought the whole idea about thermal storage batteries is to control the heat? Given enough insulation it is possible to store the heat when you can generate it and release it when you need it, otherwise it's just a heater.

 

[Mart Hale]

I have been looking at various ways of insulation. Perlite, straw + cob, all seem to be ways to get the job done.

I have also been looking at using something called "Styro crete" which is air crete + styrofoam. I used a lawn mower to chop up a huge amount of styro foam,

Like you I have been looking for the most economical way of pulling this off. I have even started asking the question, why not bricks instead of sand? I love sand because of the extreme high temps that can be stored in it. In my testing I did a small pail of sand over 1,000 deg. But my present setup has pex pipe which will not take those temps, but.... It is my intent to see what a trash can full of sand heated with elements will yeild.

I have also considered simple air insulation, say barrels within barrels......

I now have a trash can that I intend to fill with sand and put that inside a 55 gal barrel, and that I am considering wrapping in foam, but I don't know yet exactly what I will do, I will let my testing results lead my way.

 

[Mart Hale]

One thing that I want to test is to take a dryer vent hose and run it thru the sand...... That might be the way to move from my rocket stove to heat the sand....

 

[Hedges]

I think you could release heat by pushing air through a tube coiled at top of sand.
Tube entering/exiting bottom should minimize convection.

Sand vs. brick, convection should transfer heat faster.

Sand is most easily heated with resistance elements, so "just" 100% efficiency.
Compare to batteries, which could then power a heat pump, in some climate conditions.

 

[MyK3y]

One thing that I want to test is to take a dryer vent hose and run it thru the sand...... That might be the way to move from my rocket stove to heat the sand....

I was thinking of 1-2" pipes through the sand as a conduit for the warm air.

So my drum has a plate with a bunch of holes in a circle, that sits on a couple of bricks to give an air gap at the base, pipes are press-fitted into the holes and up through the sand, sand and the element sits on that, then there's a corresponding plate on top of that for the pipes to locate in.

The sand can be hard-packed without restricting airflow. The vent blower pulls hot air through the pipes, rather than my original idea which was a drum within a drum with the outer drum being river gravel and pulling the air through that. Hard-packed sand has a much higher heat capacity than loose-packed river gravel.



I'm not much of a one for metalwork, but there's a local CNC shop down the road from me that quoted $25 for a insert plate for the barrel cut on their plasma cutter.

I have a laser and small wood CNC, so am already set for creating the vector file - that can be a significant expense when they're paying their CAD guy $80/hr. I would just get the rough cut plate and finish it with the grinder Drill 4 holes around the drum and fit bolts to sit the plates on, seal in place with refractory cement, same as the pipes - push fit and sealed with cement, which allows easy disassembly, should I need it.

My next door neighbour will cut the vent holes for me with his small plasma. Fill them with fibreglass fill -I have a bunch left over from my workshop wall lining - which insulates radiant heat while allowing the free flow of hot air.

 

[brum]

1m3 of sand and water compared - from an ambient temp of 15C to max (assume max of 500C for sand):

H2O - you can store 85C x 4185J x 1000kg Joules before your water starts to boil = 355,725,000J - 98kWh
Sand - you can store 485C x 835J x 1600kg Joules = 647,960,000J - 180kWh

Did you calculated the losses? 500C with heat insulation will lose a lot more energy than 100C with the same heat insulation. Roughly 5 times more.

 

[MyK3y]

Did you calculated the losses? 500C with heat insulation will lose a lot more energy than 100C with the same heat insulation. Roughly 5 times more.

No, I haven't.

I think you may be looking at it wrong, though - energy storage for hot air transfer in a solid medium is very different to energy storage in a fluid or PCM.

Things like mass, heat source location, heat transfer rate, length of time of storage will factor in.

A ~1000W heater isn't going to raise 350KG of sand to 500C in 6 hours. Close to the element it may reach 500 and greater, but the sand itself will act to slow the rate of temperature normalisation through the sand, and I'm tapping it the whole time. Unlike a liquid where the whole body of water will tend to raise to the same temperature, solid-state storage has high localised temperatures, reducing in inverse square ratio the further from the heat source. If I left it a week without tapping energy off, I'm sure I'll lose a lot, but I'm not making a municipal sand battery, it's a hot-air heater for domestic use.

I've got three thermocouples so I'll get a good idea of losses once it's up and running and if necessary, can address higher level of insulation at that time.

A strandboard box lined with rockwool or fibreglass batts will be more than good enough, IMO.

If you look at the video of the Finnish municipal sand batteries there's a segment they filmed in IR. The building is clearly much hotter than the surrounding area. If they're not that bothered about extreme insulation, neither am I.

 

[orangezero]

Underneath and outside.

The cost for a water-based system far exceeds that of ducted air sand battery and it is a whole lot more work. Our whole underside of the house is insulated using between-joist rockwool about a foot thick then covered in a heavy PVC membrane stapled to every joist. You would need to take all that out.

Not the way I'm proposing it isn't, but if you can't access underneath the floor it would complicate things for sure. My issue with air sand is you have to still somehow move the heat around with fans. I have just heard too many people gushing about radiant floor heat and I've liked it the places I've stayed that had it.

I am pretty happy with a sand battery and ducted warm air. The sand battery itself is about $50 and $40 of that cost I already have. All I really need is a 44Gal drum and they're $10. To make a test-bench working system I would need an inline fan and a floor duct kit - $150-200.

I like your style. That 44gal drum is also designed to store water. I'm talking $20 DC water pumps, which are comparable to costs of fans. The thing I like about a diy hydronic system is nothing has to be made for high heat as the water running through the pipes is only around 38C. A bigger tank can store just as much energy as a smaller one at higher temps. Higher temps concern me to some degree.

Dont assume 'Pex tubing isn't that much' - we are an island, remote from anywhere, and things are expensive here. Anything plumbing related is expensive.

Edit: just checked - 150ft of 1/2" is $170 (well, 50M of 20mm, but you guys don't do metric). For the same money in the US you get 600ft - 4x the length or 1/4 of the price.

I do metric, makes sense to me I just got a ton of 40mm x 40mm aluminum extrusion for almost nothing because my fellow Americans don't know what that means and ignored the listing. I actually think Pex is expensive, but I've found people selling unused leftovers for amounts I could afford. One guy was almost giving away 500ft rolls of 10mm plastic tubing, but that is pretty small diameter and probably wouldn't work great so I haven't tried it. Even then, I wouldn't need a single pex connector with my idea. I agree the traditional way of doing hydronic systems is expensive. I haven't quite figured out all the non-pex tubing options that are available. And I'm not totally sold on water just because a single leak could definitely make a disaster whereas hot air would be okay.

And while we're talking prices - we pay > NZ$1/W for premium Mono/half-cut PV panels which is hella competitive with the US - but then we pay through the nose for inverters and electricians to install them.

The 445W panels I just got priced are NZ$350/panel for Trina Vertex S+ - USD$205. I can't find them for anything like that in the US.

I haven't looked at panels of that size since my electronics require 60 cell panels, but I can drive an hour and get used trina 330w panels for $110 before any negotiation. The most interesting thing I've seen lately are new 200w bifacials for $140, which would quality for some tax incentives. It still feels like used panels for $0.20-30/watt is a better deal overall than the tax incentives.

With expensive inverters and electricians... Have you ever seen people who directly hook panels to each other? They put a panel inside their house, disable one of the diodes so energy flows through 40 of the 60 cells, and then connect it to two 60 cell panels outside. For zero electronics cost this puts the panels very near their max power point and you mostly optimize energy production. It isn't a battery of any kind, but the inside solar panel (the mass of glass, aluminum, etc) gets up to 40-50C and heats your house. It doesn't sound perfect, but it also sounds simple and long lasting and automatic. If you do it right it can't get any hotter than panels get on a hot roof in the summer. I'm sure lots of people don't have places to put panels inside their house, it is a little strange... It seems like a decent use of cheap solar panels

 

[orangezero]

Sure. But how are you insulating that? Or are you making a passive thermal store that will just give up its heat naturally?

I thought the whole idea about thermal storage batteries is to control the heat? Given enough insulation it is possible to store the heat when you can generate it and release it when you need it, otherwise it's just a heater.

In my thought experiment, a water tank heats up and there is some energy storage there, but the water tubes heating the floor would make the mass of the wood floor the actual battery storage. My thinking is you don't need to "release it when you need it" if your house is already warm. I could envision some variability in water flow over time, and being able to manual adjust it, but ideally it would just distribute the heat over time and it wouldn't be something I think about that much. But I'd also have my old backup furnace that I'd have available and use only when the sun wasn't providing enough energy.

Another idea for heating your sand cheaply and less at a single point in the sand is to use a long length of thin wire covered with a high temp silicone coating. You need to get the resistance right, but in this case you are using the "high amps heat up thin wire" as a positive instead of something to be avoided like most typically think of it. I've even seen people placing long runs of wire embedded in their concrete to heat their slab. Again, low part count, potentially long lasting, could be semi-autonomous and not requiring an additional inverter or electrician involved.

 

[MyK3y]

Not the way I'm proposing it isn't, but if you can't access underneath the floor it would complicate things for sure. My issue with air sand is you have to still somehow move the heat around with fans. I have just heard too many people gushing about radiant floor heat and I've liked it the places I've stayed that had it.



I like your style. That 44gal drum is also designed to store water. I'm talking $20 DC water pumps, which are comparable to costs of fans. The thing I like about a diy hydronic system is nothing has to be made for high heat as the water running through the pipes is only around 38C. A bigger tank can store just as much energy as a smaller one at higher temps. Higher temps concern me to some degree.


I do metric, makes sense to me I just got a ton of 40mm x 40mm aluminum extrusion for almost nothing because my fellow Americans don't know what that means and ignored the listing. I actually think Pex is expensive, but I've found people selling unused leftovers for amounts I could afford. One guy was almost giving away 500ft rolls of 10mm plastic tubing, but that is pretty small diameter and probably wouldn't work great so I haven't tried it. Even then, I wouldn't need a single pex connector with my idea. I agree the traditional way of doing hydronic systems is expensive. I haven't quite figured out all the non-pex tubing options that are available. And I'm not totally sold on water just because a single leak could definitely make a disaster whereas hot air would be okay.



I haven't looked at panels of that size since my electronics require 60 cell panels, but I can drive an hour and get used trina 330w panels for $110 before any negotiation. The most interesting thing I've seen lately are new 200w bifacials for $140, which would quality for some tax incentives. It still feels like used panels for $0.20-30/watt is a better deal overall than the tax incentives.

With expensive inverters and electricians... Have you ever seen people who directly hook panels to each other? They put a panel inside their house, disable one of the diodes so energy flows through 40 of the 60 cells, and then connect it to two 60 cell panels outside. For zero electronics cost this puts the panels very near their max power point and you mostly optimize energy production. It isn't a battery of any kind, but the inside solar panel (the mass of glass, aluminum, etc) gets up to 40-50C and heats your house. It doesn't sound perfect, but it also sounds simple and long lasting and automatic. If you do it right it can't get any hotter than panels get on a hot roof in the summer. I'm sure lots of people don't have places to put panels inside their house, it is a little strange... It seems like a decent use of cheap solar panels

I get the worry about high temps - my house is all timber construction, other than the concrete piles. The nice part about a sand battery in a 44Gal drum - sand and steel doesn't burn until you hit 1800C

Solar is relatively new here so second hand market is limited. We also have zero rebates or subsidies or incentives, so there's nothing much second hand at a big saving over new. So I may as well get new. I'd bite your hand off for 330W Trina's for $110. I could convert my workshop to solar for that price.

The electrician price isn't so bad for me - I do everything up to the final AC/meter connection, so only need a registered sparky for a few hours.

We already have some radiant underfloor heating using pads that you fix between the floor planks and the underslung insulation, but it's proven unreliable. Chinese, from Aliexpress, it lasted a few months, but I'm not going to pursue the tech. I've also got underfloor heating in the bathroom from the same suppler - failed within a day. Now I've got to pull up all the tiles, remove the heater coils, mortar, and re-lay the whole thing. I will buy a reputable brand this time. At least I don't need an electrician - that side of it has been completed.

 

[upnorthandpersonal]

In my thought experiment, a water tank heats up and there is some energy storage there, but the water tubes heating the floor would make the mass of the wood floor the actual battery storage. My thinking is you don't need to "release it when you need it" if your house is already warm. I could envision some variability in water flow over time, and being able to manual adjust it, but ideally it would just distribute the heat over time and it wouldn't be something I think about that much.

Ideally you would want a concrete floor insulated from the bottom and disconnected (insulated, i.e. floating) from the foundation. That's how I do it. I have a 3000L water tank for storage (source: heatpump and wood burner), and the mass of the concrete floor. I have a controller that decides how much warm water needs to flow into the floor to keep the temperature constant (weather compensated). The 3000L water also provides the domestic hot water as well. The delta between in incoming and outgoing water in the floor heating should be small and the incoming water should be as low temp as possible to minimize losses and for best efficiency.

This is also combined with a masonry fireplace (read: a large mass of store about 2000kg , wood fired, holding about 60kWh of heat) to keep the place warm with very little effort, day and night even in the harshest f winters. In case anyone want to read up on the details:

#6 Heating Systems

In a cold climate, effective and efficient heating systems are paramount. Because I’m off-grid, I also want a heating system that is fail…

medium.com

 

[MattiFin]

A ~1000W heater isn't going to raise 350KG of sand to 500C in 6 hours. Close to the element it may reach 500 and greater, but the sand itself will act to slow the rate of temperature normalisation through the sand, and I'm tapping it the whole time. Unlike a liquid where the whole body of water will tend to raise to the same temperature, solid-state storage has high localised temperatures, reducing in inverse square ratio the further from the heat source. If I left it a week without tapping energy off, I'm sure I'll lose a lot, but I'm not making a municipal sand battery, it's a hot-air heater for domestic use.

No free lunch here, at least on small scale like less than 800 tons of sand.
Either the thermal conductivity of the sand limits the rate of energy extraction/storing and you are not utilizing the whole mass or you don't get the benefit of sand acting as a insulation layer.

 

[MattiFin]

If you look at the video of the Finnish municipal sand batteries there's a segment they filmed in IR. The building is clearly much hotter than the surrounding area. If they're not that bothered about extreme insulation, neither am I.

I wouldn't get too exited over one experimental sand battery.
We have water storage tanks in real-world use and in realistic capacities:For example Mustikkamaa with 260 000 m3 district heat hot water storage and incoming project in Kuusikko with 1 000 000 cubic meters and 90 000 MWh storage capacity.

 

[upnorthandpersonal]

We have water storage tanks in real-world use and in realistic capacities:For example Mustikkamaa with 260 000 m3 district heat hot water storage and incoming project in Kuusikko with 1 000 000 cubic meters and 90 000 MWh storage capacity.

Here is some info in English on the one in Vaasa:

Storing heat and power - EPV

www.epv.fi

 

[MyK3y]

Another idea for heating your sand cheaply and less at a single point in the sand is to use a long length of thin wire covered with a high temp silicone coating. You need to get the resistance right, but in this case you are using the "high amps heat up thin wire" as a positive instead of something to be avoided like most typically think of it. I've even seen people placing long runs of wire embedded in their concrete to heat their slab. Again, low part count, potentially long lasting, could be semi-autonomous and not requiring an additional inverter or electrician involved.

I've thought about that, but the potential temps are over 500C, even sheathed cable will degrade fairly quickly. Stovetop elements are very tough and cheap - I'm not sure I could find anything DIY as good or reliable for such a good price.

As to electricians, we need them for anything that connects to the grid - so anything low voltage, anything DC, anything not connected to the house AC and thus to the grid, I can do. It's more about keeping the grid safe for the rest of the users - we're pretty good with not being overly-officious in NZ. No need for permits and such for most things, but if you screw up - it's on you. We intend to eventually go off-grid, at which time I can do what I like.

The other reason for a 'separate infrastructure' sand battery is that if it all goes to custard I can just attach the two panels into my home solar and I've lost some time and maybe $500 in parts - which I am sure I can reuse.

I've been looking at 'sauna elements' - these are big squarish elements of 2500W at 110V, and I can get close to 110V from a couple of series-wired panels. They're ~NZ$40 on aliepress. Of course they're not really '110V or '220V', that's just the rating at that voltage - so 2500W at 110 would be maybe 3500 at 220, with the inverse proportional nature of resistance - I suspect that a '110' just has a thinner resistor than a '220'

While looking, I found some more interesting elements - straight ones of ~500W that could be arrayed, as well as customisable ones. Those 3000W ones at the lower-right are $43. I'm going to order a couple.





And, ones you may be interested in - spa/pool water heaters. They go up to 15kW! Just set the temperature and off it goes. You still need a pump and it monitors water flow, so safety, 1.5" connections - set and forget.

 

[MyK3y]

No free lunch here, at least on small scale like less than 800 tons of sand.
Either the thermal conductivity of the sand limits the rate of energy extraction/storing and you are not utilizing the whole mass or you don't get the benefit of sand acting as a insulation layer.

I doubt I will be utilising the whole mass until I'm taking less than I'm putting in. I'm heating a lounge in a temperate country in the South Pacific where the lowest temp we've seen in the last couple of years is 5C. Not -5C, 5C. That's summer where you are, isn't it?

We, as a country, are transitioning away from fossil fuel and non-renewable sources - so no oil-fired, coal or wood-fired source of energy. I want to get rid of our wood-burner as it takes up too much room in a small house, is dirty, and adds significantly to my carbon footprint.

Think of it more along the lines of 'Perfect is the Enemy of Good' - If I get to store some clean energy in sand, cheaply and safely, and get to duct warm air around my small (80m2) house, for the investment of a few hours and <$1000, I'll be very happy.

It's the 2nd week of spring - the sun is shining brightly and it's 11C at 8am. In the middle of winter - June - it averages 12-14C at lunchtime, but you can get days of 18C. We don't light the fire every day, even in winter - even though made of wood and with single-pane windows, our houses are reasonably well insulated. Mine has brick-clad timber walls around 350mm thick, but it is nearly 100 years old so there are gaps and draughts which probably do more to affect the temps than the loss of a few kJ of stored heat. The biggest draught source is the wood-burner and its flue. It's windy today and I can hear the wind whistling up the flue.

As with anything, moderation and acceptance of less-than-perfection is the key to a happy life.

And, unlike in Finland, if the power goes out or we have no heating at all, we won't die - we'll just be uncomfortable for a while. When I'm working at home and don't need to be on the tools, I often retreat to the bedroom for a few hours, rather than waste fuel heating the whole house.

 

[MyK3y]

I wouldn't get too exited over one experimental sand battery.
We have water storage tanks in real-world use and in realistic capacities:For example Mustikkamaa with 260 000 m3 district heat hot water storage and incoming project in Kuusikko with 1 000 000 cubic meters and 90 000 MWh storage capacity.

Sure - but this thread is about sand energy storage, not water - is there a water thread?

Why do all the sand storage threads end up with blokes talking about water?

 

[orangezero]

"All goes to custard" made my day. I've never heard that before. Thanks.

My high temp concern is certainly more for water because runaway could mean steam and pressure and problems.

I've seen people discussing various heating elements can have plastic which melts well before the limit of the metal parts. Make sure you aren't getting one of those designs.

Maybe there are lots of reasons, but why wouldn't you just get a roll of kanthal or nichrome wire and lay it out as the length you want directly in the sand? What function does any of the metal in those heating elements actually do in your specific situation? Normally they'd provide some structure and support and perhaps help radiate the heat somewhat. In your case the sand could be doing all of that

I also forgot to mention earlier, I think you said maybe straw insulation and cob. I always tended to prefer lime plaster instead of cob on strawbales. I think it allows moisture to pass through the wall a bit better but it has been a while since I've read about that. I know there were a fair number of houses in Nebraska made of straw. It seems like a really nice way to make a well insulated area if you have a cheap supply of straw.

 

[MyK3y]

"All goes to custard" made my day. I've never heard that before. Thanks.

A well-used epithet in my part of the woods

My high temp concern is certainly more for water because runaway could mean steam and pressure and problems.

I've seen people discussing various heating elements can have plastic which melts well before the limit of the metal parts. Make sure you aren't getting one of those designs.

Yep - that's why I want to use stove-top elements. They run 800C-950C in air.

Maybe there are lots of reasons, but why wouldn't you just get a roll of kanthal or nichrome wire and lay it out as the length you want directly in the sand? What function does any of the metal in those heating elements actually do in your specific situation? Normally they'd provide some structure and support and perhaps help radiate the heat somewhat. In your case the sand could be doing all of that

Stove elements are tough - built to take years of metal pans banging around. Nichrome wire is what is inside them and is a little fragile - I have spools of Nichrome.

I also forgot to mention earlier, I think you said maybe straw insulation and cob. I always tended to prefer lime plaster instead of cob on strawbales. I think it allows moisture to pass through the wall a bit better but it has been a while since I've read about that. I know there were a fair number of houses in Nebraska made of straw. It seems like a really nice way to make a well insulated area if you have a cheap supply of straw.

Not me.

 

[Mart Hale]

Another thought........

What if we took the water tank from this system and replace it with oil......

Then use the heat pump to heat the sand by having the oil tank in the center of a sand pile...


From what I have heard heat pumps are more efficient than heating coils....

System is like $5,000, but I like it uses CO2 for the refrigerant.... Wild ideas but while I am thinking outside the box why not break all boxes ;-)